When an electronic system that contains several functional units with variable power consumption is to be supplied with power and the capacity of the power source is limited, then a limit value may be preset for the total power consumption which would be exceeded if all functional units of the system were to be supplied at their highest possible individual power consumption. Then there is the problem of distibuting the electrical power according to the operating status of the functional units in such a way that functional units with a higher priority do not experience any impairment in operation whereas a reduction in power can be accepted temporarily for functional units of a lower priority. A limit on the total power supply may be due to the fact that power supply equipment intentionally delivers only limited power, or the power may be distributed over comparatively thin and long lines.
One example of an electrical system in which several functional units are to be supplied with at least partially variable power consumption and at the same time a limit value for the total power is given is a so-called "intelligent" telephone subscriber's station that is supplied over subscriber's lines from the central battery of a telephone system. Such a telephone subscriber's station contains control circuits that have relatively low power demand because they are usually in the form of microprocessors. On the other hand, they also contain functional units with a higher power demand that may contain amplifiers for telephone receivers and microphones plus, optionally, those for speakerphones. Such functional units consume different amounts of electricity depending on their modulation, and this change in demand may result in outage of the controlling functional unit due to insufficient power supply in cases of limited total available power. Thus, there is a need to maintain the power supply to functional units with a higher priority in any case and not impair them due to transient high-power demand by functional units of a lower priority.
In order to meet this requirement, the functional units may be dimensioned in such a way that the sum of all highest possible individual powers corresponds to this total power supply even with a maximum feeder line length, i.e., with minimum total available power. However, this has the disadvantage that the dimensions of the circuit must be based on maximum line length, and thus the available power supply is not fully utilized when such a system is used with short line lengths. It is also possible to maintain the operability of functional units that have the highest priority and which must always operate reliably regardless of the operating status of other functional units for a brief period of time without power consumption over the feeder line. Such a solution has the disadvantage that mechanical two-way contacts and/or additional local power sources must be provided. In the wake of the development of progressively more efficient telephone subscriber's stations that have relatively extensive display fields and new power features using digital technology, an increase in power supply to the corresponding functional units is unavoidable. This especially affects the controlling functional units whose power demand is then such that a temporary interruption in power consumption is not technically justifiable even with additional power sources provided.